1. Field of the Invention
The present invention relates to an acceleration sensor for use in a vehicle and for outputting a signal indicative of a command to start a collision safety system such as an air bag system and a seat belt retracting system.
2. Description of the Prior Art
There is known a conventional acceleration sensor of the type described in Japanese Patent Publication No. 19282/82 which is used for outputting a signal indicative of a command to start an air bag system upon the collision of the vehicle. This acceleration sensor includes a resinous frame, a roller capable of rolling along a guide surface of the resinous frame, a roll spring wound around the roller to constitute a movable contact, and a stationary contact mounted on the resinous frame to lie in a path of a rolling movement of the roller. The roller is arranged to roll against a biasing force of the roll spring by an inertia due to a shock thereby, causing the stationary contact to be electrically connected with the movable contact.
In these days, there is a tendency that an air bag system for a vehicle is mounted not only for a driver's seat but also for an assistant driver's seat. However, the conventional acceleration sensor is provided with only a single contact and for this reason, there is a problem in that it is necessary to mount an acceleration sensor each for the driver's seat and the assistant driver's seat separately; thereby resulting in an increase in size of the system and an increase in cost. A monitor resistor is connected to the contact of the acceleration sensor, but it is necessary differentiate to the resistance value of the monitor between cases when the above-described conventional acceleration sensor is used exclusively for the driver's seat and when it is used for both the driver's seat and the assistant driver's seat. This causes a problem in that production and maintenance costs of the acceleration sensor increase.
In addition, the roll spring forming the movable contact of such a conventional acceleration sensor is locked at one end thereof to a roll spring retaining member supported on a support wall rising from the resinous frame. However, the roll spring retaining member is mounted in proximity to an inner surface of a casing covering an upper portion of the resinous frame and hence, if the roller spring retaining member and the casing, both made of an electrically conductive material, come into contact with each other for some reason, the movable contact is short-circuited to the casing, and an electric current which should be supplied to a squib escapes to the casing. Therefore, means for preventing this is required.
Further, the conventional acceleration sensor suffers from a problem in that a mounting bracket is mounted to a resinous casing covering a body of the sensor, and a harness is placed and potted in an opening in the resinous casing; thereby, resulting in an increase in the number of parts and an increase in contour size.